External-pressure type hollow fiber membrane modules which filtrate water from the outside of the hollow fiber membranes to the inside hollow portions have various merits including the simplicity of the seal structure which separates raw water which has not underdone membrane filtration from filtrate obtained after membrane filtration, ease of operation management, etc. The greatest feature thereof resides in that the module can have an exceedingly large filtration membrane area per unit volume of the module. Because of this, the external-pressure type hollow membrane modules are being increasingly applied in recent years to water treatment processes for producing industrial water or tap water from river water, lake water, groundwater, seawater, household wastewater, or industrial wastewater.
When raw water is membrane-filtrated using the hollow fiber membrane module, substances which are contained in the raw water and are to be removed, such as suspended substances and organic matters, accumulate on outer surfaces of the membranes to cause a membrane clogging phenomenon. As a result, the filtration resistance of the membranes increases, shortly rendering the filtration impossible. Consequently, a general method for maintaining the membrane filtration performance is to periodically stop the membrane filtration and conduct physical cleaning of the filtration membranes.
Usually, the filtration step and the physical cleaning step are automatically conducted repeatedly. Examples of the physical cleaning include air scrubbing in which air is introduced into the lower part of the membrane module to oscillate the membranes in water to thereby shake off the suspended substances adherent to the outer surfaces of the membranes, back-pressure washing (back-washing) in which water (washing water) such as filtrate is forced into the membranes by pressure in the direction reverse to the filtration direction of the membrane module, i.e., from the hollow portion-side to the outer surfaces of the membranes, to remove the suspended substances adherent to the membranes, and air/back-pressure simultaneous washing in which the air scrubbing and the back-pressure washing are conducted simultaneously, etc.
The hollow fiber membrane module is generally configured of a cylindrical case and a hollow fiber membrane bundle housed in the cylindrical case. One end of the hollow fiber membrane bundle is fixed to one end of the cylindrical case by casting one potting material and the other end of the hollow fiber membrane bundle is fixed to the other end of the cylindrical case by casting the other potting material. The hollow fiber membrane bundle used here is usually configured of a bundle of hundreds to tens of thousands of hollow fiber membranes.
In the case of external-pressure type hollow fiber membrane modules, the one ends of the respective hollow fiber membranes fixed by the one potting material have openings configured of the hollow portions opening at an outer surface of the one potting material. The filtrate obtained by passage and filtration of the raw water through the hollow fiber membranes flows through the hollow portions to reach the openings and the filtrate which has passed through the openings flows to a filtrate outlet provided to the cylindrical case. The other ends of the respective hollow fiber membranes fixed by the other potting material usually have closed portions formed by sealing of the hollow portions inside the other potting material.
Well known as methods for the fixing by casting a potting material are a stationary method and a centrifugal method. The stationary method is a method in which a liquid potting material, e.g., resin having flowability, is fed with a constant delivery pump or the like from below the hollow fiber membrane bundle and the potting material is solidified (cured) at or near tip ends of the respective hollow fiber membranes. The centrifugal method is a method in which a liquid potting material is moved to the end of the case by centrifugal force and solidified (cured). The solidified potting material serves as a fixing member for fixing the respective hollow fiber membranes to the inside of the potting material and fixing the hollow fiber membrane bundle to the cylindrical case.
In either method, however, the resin having the flowability creeps up along the outer surfaces of the hollow fiber membranes to a height of about several millimeters to several centimeters at the interface between the hollow fiber membranes and the potting material to form an uneven resin surface and is solidified in this state. In the case where the hollow fiber membranes which have been fixed to the resin (fixing members) solidified in such a state receive oscillations generated during air scrubbing, local stress is imposed on the hollow fiber membranes at the uneven resin surfaces, resulting in hollow fiber membrane rupture in some cases.
To an upper side face of the cylindrical case housing the hollow fiber membrane bundle, a nozzle used for discharge of circulating water and wastewater is provided. The resin surface (lower face of the upper fixing member), on an inner side with respect to an axial direction of the cylindrical case, of the upper potting material (upper fixing member) is formed at substantially the same height position as an upper end of an inner flow path of the upper discharge nozzle so that the circulating water and the wastewater can be discharged efficiently.
If the resin surface (lower face of the upper fixing member), on the inner side of the cylindrical case, of the upper fixing member fixing the respective hollow fiber membranes is disposed above the upper end of the inner flow path of the upper discharge nozzle, air or suspended substances may stay in the space between the lower face of the upper fixing member and the upper end of the inner flow path of the upper discharge nozzle, resulting in degradation of turbidity removing performance and reduction in an effective membrane area of the hollow fiber membranes for filtration in this case.
Around the upper portion of the hollow fiber membrane bundle, a flow straightening cylinder having flow straightening holes is disposed and a discharge flow path communicating with the upper discharge nozzle is formed.
In order to further enhance the air scrubbing effect, Patent Document 1 proposes provision of slack to hollow fiber membranes housed in a cylindrical case. By fixing the opposite ends of the hollow fiber membranes to the bonded/fixed portions at opposite ends of the hollow fiber membranes while providing the slack to the hollow fiber membranes between bonded/fixed portions, the hollow fiber membranes oscillate to proper degrees during the air scrubbing and effective scrubbing can be conducted. However, during water discharge at a high flow rate, the hollow fiber membranes pushed by the discharged water and the air flow close the flow straightening holes to thereby cause a pressure loss or the hollow fiber membranes are pushed against the flow straightening holes and damaged.
To solve the problems, Patent Document 2 proposes a hollow fiber membrane module in which grooves connecting flow straightening holes are formed on an inner face of a flow straightening cylinder provided between a water discharge nozzle and a hollow fiber membrane bundle.